This invention relates generally to nonwoven tubular filters. In one aspect, the invention relates to the manufacture of nonwoven tubular filters in which nonwoven fibers are deposited on a rotating mandrel. In another aspect, the invention relates to the withdrawal of the tubular filter formed on a rotating mandrel.
Nonwoven fabric, particularly meltblown fabric, have long been used as filters. Because of the microsize of the filters (i.e. microporosity) and the random collection of the fibers, the meltblown fabrics exhibit excellent properties of filtration at reasonable pressure losses. Meltblown fabrics are generally made by forming a web which can be readily converted to a plainer filter. For example U.S. Pat. 4,714,647 discloses the manufacture of meltblown filters, wherein fibers of the same composition but different sizes are collected on a horizontal plane and used as a multi-layered filter.
It is difficult to convert meltblown webs into tubular filters because of the need for tubular filters to be continuous without a bonding seam. For example, if the web disclosed in U.S. Pat. Nos. 4,714,647 were rolled into a tube, a seam would be required to secure the inner and outer edges to the tube body.
Efforts have been made to form the meltblown web directly into a tube continuously on a mandrel. For example, in U.S. Pat. Nos. 3,933,557 and 4,032,688,meltblown fibers are deposited onto a rotating mandrel forming a tube which is continuously withdrawn and cut to proper length for the filter. The meltblowing die is positioned at an angle with respect to the axis of the mandrel so that a variation in density of the fibers is achieved as the tube moves longitudinally on the mandrel through the zone of fiber deposition.
U.S. Pat. Nos. 4,112,159 and 4,116,738 also disclose the deposition of meltblown fibers onto a rotating mandrel. The apparatus disclosed in these patents, however, deposit the fibers onto a core so that the final tubular filter comprises the core and the fibers wound about the core.
U.S. Pat. No. 4,021,281 discloses the deposition of meltblown fibers onto a rotating drum to form a relatively large diameter tube which is then flattened thereby forming a two-layer web. The web then is wound about a core material forming a tubular filter. The filter, however, is not continuous.
U.S. Pat. No. 4,594,202 discloses an apparatus for forming tubular filters by depositing meltblown fibers onto a drum which are then rolled onto a mandrel.
As discussed in U.S. Pat. No. 4,714,647, in many filters it is desirable to have a variable fiber size gradient across the filter. The fiber size gradient across the filter (i.e. in the direction of fluid flow) combines the filtration efficiency of each filtration layer. As described in U.S. Pat. 4,714,647, the first layer comprises meltblown fibers having large fiber size, the intermediate layers have medium fiber size, and the-final layer has small fiber size. The pore size is a function of fiber size so that the filtration of the laminate described above would result in the separation of large particles in the first layer, medium size particles in the second layer, and finally the smallest particles in the final layer.
The same filtration principles would apply in tubular filters. However, it has been difficult to provide a continuous filter having a fiber size gradient across the radius of the filter.